dart-grpc: Raise Dart SDK requirement

We no longer support Dart 1.x.

Author: mit-mit

docs/quickstart/dart.md

@@ -20,7 +20,7 @@ if you encounter any.</p>
 
 #### Dart SDK
 
-gRPC requires Dart SDK version 1.24.3 or higher. Dart gRPC supports Flutter and Server platforms.
+gRPC requires Dart SDK version 2.0 or higher. Dart gRPC supports Flutter and Server platforms.
 
 For installation instructions, follow this guide: [Install Dart](https://www.dartlang.org/install)